Resistance Fighter: When Weeds Fight Back | Syngenta US

Managing Herbicide Resistant Weeds

as seen in progessive farmer logo

Growers return to the basics to fortify their tactics against herbicide-resistant weeds.

By Elton Robinson

Spray Nozzles

During a typical growing season, cropland around central Illinois can offer up stark contrasts in weed control. On one side of the blacktop, a soybean field can be so overrun with herbicide-resistant waterhemp, it’s hard to see the crop. Directly across the road, soybeans can be clean as a whistle.

Weed scientists at the University of Illinois who made daily jaunts down these roads wondered what was causing these big gaps in weed control. Their curiosity evolved into a far-reaching study of landscape, soil, weed and farm-management data from 105 central Illinois grain farms, including almost 500 site years of herbicide application records, collected between 2004 and 2010.

The researchers found that herbicide management practices were the most important factor in the development of resistance in those fields. According to weed scientist Aaron Hager, a coauthor of the study, the occurrence of glyphosate-resistant waterhemp was greatest in fields where glyphosate had been used in over 75% of the seasons included in the analysis, where fewer modes of action were used each year, and where herbicide rotation occurred annually.

Furthermore, the differences between waterhemp-infested and clean fields came down to a single management practice. The best predictor of whether glyphosate resistance was in a field had to do with whether a farmer used overlapping effective modes of action, Hager says. “It could be using tank mixes or premixes, but the key is that you have to have at least two herbicides that are effective.”


Unfortunately, always applying two effective modes of action has become a lot more challenging with the development of highly competitive weeds resistant to multiple groups of herbicides.

“To explain what we’ve found about always using two effective modes of action is easy,” Hager says. “To actually implement it is not. If you have glyphosate-resistant soybeans, your options for postemergence control are glyphosate, PPO inhibitors and ALS inhibitors. ALS inhibitors are functionally eliminated because of how pervasive resistance is. If your weeds are also resistant to glyphosate, all you’re left with is PPOs.”

At the same time, many producers are learning or relearning the nuances of tank mixing, says Purdue University weed scientist Bill Johnson. “Prior to glyphosate-tolerant crops, producers did a great job of weed control and resistance management because they were using narrow spectrum herbicides and they had to tank mix more. Today, we have a generation of farmers who are having to get back to the basics of how herbicides work—things like mixing order and what adjuvants to put in the tank.”

Spray Stack Filling


The experts stress not to depend too much on one herbicide to control weeds. A good example of this approach comes from farmer Dustin Edwards, Lebo, Kansas, who has never relied heavily on glyphosate as his primary herbicide—even though most of his crops are glyphosate tolerant. “Glyphosate is a backup,” he stresses.

In the spring, Edwards focuses on preemergence herbicides and residual control in his corn. He might use a premix of acetochlor, clopyralid and flumetsulam, which contain three modes of action. “I can add atrazine for $3 an acre,” he explains.

Weed scientists also urge producers to add as many nonchemical tactics to their weed-control toolbox as possible, including tillage when appropriate, cover crops, narrow rows and crop rotation.

Most importantly, don’t allow weed seed to enter the soil seed bank. Keep in mind, weed scientists say, one of those seedy characters in your soil might be resistant to your most effective herbicide.

Here’s a closer look at some of the most competitive herbicide-resistant weeds in the South and Midwest, and suggestions on what to put in the tank to manage them.

Icon Marestail

Marestail: Hard to Manage

Marestail-management strategies rely heavily on burndown treatments that include six to eight weeks of residual control from preemergence herbicides and the use of soybeans with herbicide traits other than glyphosate resistance, such as glufosinate-resistant soybeans. Most marestail is resistant to glyphosate and ALS herbicides.

A fall burndown application for marestail can reduce variability from a spring-only application, according to Christy Sprague, weed scientist at Michigan State University. The fall application should be applied when marestail is at the rosette stage using 2,4-D, dicamba or saflufenacil as the base herbicides. Tank mixes with other herbicides (glyphosate) will be needed to control winter annual and perennial weeds.

Spring burndown should be made on marestail less than 4 inches tall. Sprague suggests tank mixes of 2,4-D and glyphosate; 2,4-D, paraquat and metribuzin; 2,4-D, saflufenacil and glyphosate; glufosinate and metribuzin; or saflufenacil and glyphosate or glufosinate. Sprague says the best residual control of two-way-resistant marestail are tank mixtures or premixtures that contain two non-ALS herbicides, such as metribuzin and flumioxazin or sulfentrazone.

“We’re seeing that marestail is continually emerging throughout the growing season,” Sprague adds. “No-till soybean producers should consider a strong burndown product with a residual and planting glufosinate-resistant or dicamba-resistant soybeans.”

Most postemergence options for marestail are available in corn, according to Gordon Vail, herbicide technical product lead for Syngenta. In glyphosate-resistant corn, “you can use a premix of S-metolachlor, mesotrione and glyphosate and add atrazine or a premix of dicamba and diflufenzopyr. A premix of bicyclopyrone, mesotrione, atrazine and S-metolachlor or a premix of mesotrione, S-metolachlor and atrazine-applied preemergence will also kill emerged horseweed.”

Icon Waterhemp

Waterhemp: Three-Way Resistance

ALS- and glyphosate-resistant waterhemp is now on the majority of Iowa corn and soybean fields, says Bob Hartzler, weed scientist with Iowa State University. “Waterhemp resistance to ALS inhibitors, glyphosate and atrazine is more common in southern Iowa. Three-way resistance to glyphosate, ALS inhibitors and PPO inhibitors is also common, and that’s causing headaches because we’re running out of options. Less common but increasing are waterhemp populations resistant to four and five herbicide groups.”

Another issue for Iowans is that waterhemp seed banks have built so high that they can’t be managed with a postemergence program alone, Hartzler adds. The large seed banks also increase the likelihood of new herbicide resistances evolving.

Regardless of the resistant traits in the crop, Hartzler says growers should design weed-control programs that take pressure off the postemergence herbicides, including glufosinate, dicamba and 2,4-D. “This means going heavily and frequently with preemergence products.”

Hartzler advises producers to apply preemergence herbicides at full rates, then include another residual product with the first postemergence application. “By doing that, you’re minimizing the population that is exposed to those postemergence products and reducing the likelihood that the first glufosinate-resistant or dicamba-resistant waterhemp pops up. The residual added to the post product will reduce problems with waterhemp that emerge after the post application.”

Preemergence programs can be based on either Group 14 (flumioxazin and sulfentrazone) or Group 15 (S-metolachlor, acetochlor, pyroxasulfone) herbicides, according to Hartzler. “Some producers are going back to the dinitroanilines [Group 3]. If they’re tilling anyway, they may view the low cost of the Group 3 products [trifluralin, pendimethalin] as an attractive option. The Group 15 herbicides can be applied postemergence to extend control later in the season,” Hartzler says.

Syngenta’s Vail agrees that effective management for waterhemp and Palmer amaranth should focus on preemergence. “You can use a premix of S-metolachlor and metribuzin [Boundary®] or fomesafen and S-metolachlor [Prefix®] in your soybeans or a premix of bicyclopyrone, mesotrione, atrazine and S-metolachlor [Acuron®] in corn.”

Icon Palmer Amarath

Palmer Amaranth: Schoolyard Bully

In Arkansas and much of the Mid-South, Palmer amaranth is the ultimate schoolyard bully, according to Jason Norsworthy, a weed scientist at the University of Arkansas. “We deal with any other weed issues after we deal with Palmer amaranth,” says Norsworthy.

Palmer amaranth with three-way resistance to ALS herbicides, glyphosate and PPOs are now common throughout northeastern Arkansas, western Tennessee and the Missouri Bootheel.

“Palmer amaranth is a fight, an absolute fight,” explains Belzoni, Mississippi, farmer Trey Koger. “We manage them from March 1 or earlier until November. If Palmer amaranth ever comes up in glyphosate-tolerant soybeans or cotton, it is almost impossible to effectively manage.”

The battle has gotten easier for Koger since he went to all dicamba-resistant soybeans in 2017. “Corn is also a very effective tool from a rotation standpoint to manage resistance in Palmer amaranth. You have a lot of tools in corn, and you have five weeks from corn emergence until canopy closure.”

On dicamba-tolerant soybeans, Koger applied a premix of chlorimuron and metribuzin behind the planter. He applied an approved, low-volatility dicamba product early postemergence, followed by a premix of S-metolachlor and glyphosate, followed by a second application of the approved dicamba product.

“When the soybean canopy started to defoliate, there were banged-up Palmer amaranth that were still alive,” Koger adds. “I don’t think it was a resistance issue. They had just gotten larger than we wanted when we sprayed them.” The weeds were taken out with tillage behind the combines.

Behind the planter on his Roundup Ready corn, Koger applied a premix of rimsulfuron and mesotrione, plus atrazine and glyphosate on half his corn and a premix of clopyralid, acetochlor and mesotrione, plus glyphosate on the rest. That was his only herbicide application.

Although Palmer amaranth has shown resistance to PPOs in the South, “PPO inhibitors are still extremely valuable tools,” explains Dane Bowers, herbicide resistance technical product lead for Syngenta. “Producers often tank mix S-metolachlor and fomesafen with glufosinate on glufosinate-tolerant soybeans. Even though there are PPO-resistant weeds postemergence, PPOs are still effective when applied preemergence or before weeds emerge,” he points out.

Purdue’s Johnson says to think of managing Palmer amaranth and waterhemp as a series of skirmishes. “You don’t want to allow the weeds to gain a foothold, then go out and fight a single battle and hope that you win. Continue to erode the foundation with tillage and residual herbicides so you’ll have a weaker enemy coming out of the ground,” he stresses.

Icon Giant Ragweed

Giant Ragweed: Early Riser

Giant ragweed is a concern because of its early emergence and rapid growth, Iowa State’s Hartzler says. “Many populations are resistant to glyphosate and ALS inhibitors, which makes it very difficult to manage. It’s a completely different weed from waterhemp and a lot more of a threat to yield potential.” Hartzler estimates that about 10% of Iowa farmland is infested with giant ragweed.

Delaying planting is an effective, albeit unpopular, option for producers, according to Hartzler. “Giant ragweed comes on early, and the bulk of the emergence happens within two or three weeks. If producers can delay planting soybeans until the first week of May, they may eliminate 90% of giant ragweed with seedbed preparation tillage or a burndown herbicide.”

Growers who choose not to delay planting also have a dilemma, according to Hartzler. “Producers are hesitant to clean them up two weeks after planting because other weeds have not started emerging yet. But they can’t hold off on applications because giant ragweed quickly exceeds size restrictions for post products. Timeliness of application is critical.”

Hartzler suggests planting into a weed-free seedbed and using flumioxazin or a sulfentrazone-containing products preemergence. “This won’t give you full-season control, but it is going to give you some short-term activity providing more flexibility with the post application.”

He says 2,4-D-resistant soybeans (when all approvals are ironed out) or dicamba-resistant soybeans “are excellent options for control because the growth regulators are highly effective on giant ragweed.”

Icon Italian Ryegrass

Italian Ryegrass: Tough in Corn

Corn and soybean producers should take out resistant Italian ryegrass with fall residual and spring burndown programs, says Mississippi State University Extension weed scientist Jason Bond. The biggest issue with the weed is in corn, where, uncontrolled, it can reach 12 to 24 inches in height by planting.

The fall residual can contain S-metolachlor, a premix of S-metolachlor and metribuzin, or S-metolachlor and flumioxazin for broader control. “One problem with the fall residual is that there is not a fall herbicide that selectively takes out one species,” points out Bond. “So we get into a lot of bed weathering over the winter.”

Italian ryegrass resistant to clethodim (ACCase inhibitor) in Mississippi has changed the spring burndown program for the weed, according to Bond. “In the spring, add metribuzin or atrazine to paraquat to control Italian ryegrass and other spring weeds. Italian ryegrass needs to be dead 21 days before corn planting to avoid yield loss. On soybean ground, add metribuzin or a premix that contains metribuzin to paraquat.”

Icon Common Lambsquarters

Common Lambsquarters: Few Post Options

Michigan State’s Sprague says common lambsquarters present at the time of planting should be managed with either tillage or an effective burndown herbicide application. To improve control of a burndown, add 2,4-D to either glyphosate or paraquat, she suggests.

The tank mix should include effective soil-applied herbicides, Sprague adds. “Some of the sulfentrazone-containing premixes and flumioxazin-containing premixes provide good to excellent control.”

There are few postemergence options for lambsquarters, according to Sprague. “Lambsquarters are not super susceptible to glufosinate, so generally, you have to be early on your application timing. If lambsquarters gets any size to it, it gets hard to control in that system.

“We haven’t found any glyphosate-resistant lambsquarters yet,” Sprague adds, “but there are issues controlling it postemergence. That’s probably due to environmental conditions at the time of application.”

In glyphosate-resistant soybeans, the ALS herbicide thifensulfuron has good activity on lambsquarters, notes Sprague. “But we run into problems if we have ALS-resistant lambsquarters populations.”

Icon Common Ragweed

Common Ragweed: Grower Grievances

Most producer complaints Purdue’s Johnson fields on common ragweed are about ALS-resistant populations in non-GMO soybeans. “With about 80% of our soybeans in no-till, and the fact that the ragweeds in general are early emergers, we have to get them controlled with the burndown. The good news is that common ragweed is very sensitive to 2,4-D,” Johnson explains.

Meanwhile, Sprague points out that cloransulam “was very effective for preemergence and postemergence control of common ragweed, but with ALS resistance we now see throughout Michigan, it’s becoming a nonfactor for control. From a postemergence side, we’re relying on cleanup in non-GMO soybeans with Group 14 herbicides like lactofen or fomesafen.”

Icon Barnyardgrass and Johnsongrass

Barnyardgrass and Johnsongrass: Too Tough

While barnyardgrass boasts resistance to nine modes of action, the most common populations in Mississippi are resistant to PS II Group 7 and/or synthetic auxins.

Mississippi State’s Bond believes that recently reported failures of glyphosate on barnyardgrass were likely the result of incompatibility with tank-mix partners rather than resistance. “We have not confirmed glyphosate resistance in barnyardgrass in Mississippi, but we do have barnyardgrass populations that commonly survive the treatments we use, which are typically in a tank mix of glyphosate with S-metolachlor or fomesafen.”

Bond says the reduction in efficacy due to antagonism is often only around 10% and can be hard to spot in a field.

“We can’t advocate a glyphosate-only application because the Palmer amaranth will tear us up,” Bond says. “So make sure your residual herbicide tank mix has a product that is effective on grass. Fortunately, many of the residuals we’re using to control Palmer amaranth are active on barnyardgrass, including S-metolachlor, acetochlor and pyroxasulfone.”

For Johnsongrass in corn, Bond recommends a premix of thiencarbazone (ALS) and isoxaflutole (HPPD) as a preemergence treatment. In soybeans, producers can add clethodim to their standard Palmer amaranth herbicides to pick up Johnsongrass. Glufosinate in glufosinate-tolerant soybeans will also do a good job on Johnsongrass, Bond says.

Site of Action Tool

A site of action lookup tool was developed by the Take Action pesticide resistance management initiative and funded by the United Soybean Board. The tool helps farmers quickly identify the site of action of popular herbicide brands to help ensure they aren’t using two products with the same mode of action. The site can be found at

Another good source of information is Syngenta’s Resistance Fighter: It provides education, local recommendations and a comprehensive herbicide portfolio to help growers manage resistant weeds.

Producers are starting to get the message about the importance of resistance management, according to Dave Johnson, DuPont’s product development manager for soybean herbicides and traits. “We’re seeing a lot of growers going back to preemergence products, not only to provide multiple modes of action but also to thin out the weeds they have to deal with postemergence.”

But it’s going to take more than herbicides to tame resistant weeds, Johnson adds. “Producers should not forget some of the other things they can do to manage weeds—crop rotation, scouting, keeping records of cultural practices, bringing the steel back to the field, managing weed seed at harvest, cleaning combines and using technologies that can destroy weed seed.”